Embodiments according to the present invention relate to a control circuit for a DC-DC converter (DC=direct current), which, for instance, is employed in the framework of a maximum power point tracking circuit.
Maximum power point tracking (MPPT) is a control technique that extracts a maximum value or a value close to the maximum value of a power (maximum power) from an energy source or transducer, such as solar cells or thermogenerators. The MPPT control circuit is realized in the control loop of a DC-DC converter, which is placed between a solar cell or a thermogenerator and the battery that is, for instance, the load in order to create a virtual impedance equal to the impedance of the solar cell or the thermogenerator. Nowadays, most of the common techniques of MPPT employ digital signal processors (DSPs) or micro-controllers. Simpler solutions using only analog circuits already exist, but the power consumption and complexity of these solutions is by far not optimized and unsatisfactory for many applications.
The power consumption of the MPPT circuit is typically not a constraint for its use with solar cells in outdoor applications, but may be so for the case of thermogenerators and solar cells in applications where the amount of available power is of the order of some milliwatts only. In these cases, the minimization of the power consumption of the control loop of the DC-DC converter is of special interest.
MPPT control circuits are used in conjunction with switching DC-DC converters, mostly with boost or buck-boost DC-DC converters. There are some techniques which attempt to maximize the power extracted from the power supply and other techniques which attempt to maximize the power at the load. The maximization of the power extracted from the power supply needs typically the measurement of voltage and current and their processing employing a micro-controller, a digital signal processor or an analog multiplier in order to decide which change is needed for the control signal of the DC-DC converter.
The two most famous techniques forming state of the art are the hill climbing technique and the perturb and observe (P&O) technique. The hill climbing technique involves a perturbation of the duty ratio of the DC-DC converter, whereas the P&O technique involves a perturbation of the operating voltage of the energy supply. Both techniques, the hill climbing and P&O algorithms, are based on determining the next perturbation having the same sign as the previous perturbation, when the previous perturbation led to an increase in the power extracted from the energy supply. When there is a decrease in the power caused by the previous perturbation, the following perturbation will have the opposite sign.
Analog circuits are typically implemented in ripple correlation control techniques for reaching the maximum power point (MPP). However, in this case, both voltage and current at the output of the energy supply are to be sensed and a multiplier is needed in order to determine the value of the duty cycle of the switching DC-DC converter as a function of an integral over the changing of the power ({dot over (p)}) multiplied by the change of the frequency or the duty cycle of the DC-DC converter ({dot over (v)}) over time (∫{dot over (p)}{dot over (v)}dt).
However, when a battery or another voltage source is employed at the output of the DC-DC converter, the voltage level may be considered to be fixed and it can be assumed that, by only measuring the output current, an operation at the maximum power point (MPP) is achievable. However, this control technique does not reach exactly the maximum power point, since it assumes that input power is equal to output power.
A conventional DC-DC converter with a control circuit, which measures the output current of the DC-DC converter, employs a differentiator, a comparator, a JK flip-flop and an integrator as an MPPT control loop, has previously been designed and described in [4]. For this control system to operate correctly, it is to be assumed that the control signal is below the maximum power point (MPP) and the flip-flop output state is high. Thus, the duty cycle will increase and therefore also the output current. When the MPP is passed, the flip-flop changes its state and the duty cycle decreases its value. In this case, a second flip-flop is needed which can also respond properly to the reversed situation. However, the energy consumption in relation to its operational speed is unsatisfactory for many applications, e.g. mobile devices, and for energy transducers, such as solar cells or thermogenerators, with a limited amount of power. Moreover, solutions like these often comprise a disturbing level of drift in some of the previously mentioned components.